Evidence has shown that anthrax lethal toxin (LeTx) is responsible for most disease symptoms and death due to B. anthracis infection. However, the precise molecular mechanism by which lethal toxin causes disease remains a mystery. Lethal factor (L.F), the catalytic component of LeTx, is a zinc-dependent protease that has been shown to cleave MAPKKs, thus disrupting signaling through the ERK, p38, and JNK pathways. LF is able to enter and cleave MAPKK in the various tested cell types, however, only murine macrophages were shown to be sensitive to toxin induced lysis in vitro. Furthermore, macrophages from certain inbred strains of mice are resistant to toxin, but can be sensitized if activated with bacterial components, presumably via toll like receptors (TLR). We are interested in uncovering cellular mechanisms that contribute to toxin sensitivity. Here, we propose experiments designed to elucidate TLR signaling involved in sensitization of resistant macrophages as we predict this involves events that play a role upstream of toxin activity. Additionally, we will conduct somatic cell and chemical genetic screens using a toxin sensitive macrophage cell line to identify cellular factors or processes that play a role LF sensitivity.